The present invention relates to improvements in an electric brake device, which is used for braking a vehicle, especially for braking an automobile, which generates a braking force by utilizing electric energy such as voltage and electric current.
Concerning the brake device for braking a vehicle, various types of brake devices have been developed and put into practical use. Especially, concerning the brake device used for braking an automobile, a hydraulic brake device, in which a braking force is generated by hydraulic pressure, is widely used. Concerning this hydraulic brake device, for example, a disc brake described in JP-A-55-123029 or a drum brake described in JP-A-09-100849 is widely used. A drum type caliper brake described in JP-A-59-050234 is used in some fields. In order to generate a stronger braking force, a multiple plate type disc brake device described in JP-Y-01-044820 has been conventionally proposed.
Since highly sophisticated control is applied to a vehicle recently, various types of electric brake devices, in which a braking force is generated by electric energy, have been proposed. The electric brake device is advantageous as follows. When the electric brake device is adopted, it is unnecessary to provide a master cylinder, a booster and hydraulic pipes, which need large spaces for installation, and further the manufacturing costs of which are high. Accordingly, the costs can be reduced and further the electric brake device can be easily installed. Furthermore, a period of time from when a driver has stepped on the brake pedal to when a braking force is generated can be reduced. Furthermore, sophisticated control can be conducted in order to maintain stability of running.
As an electric brake device which has been developed in the above circumstances, for example, a direct driven actuator type electric brake device described in JP-A-60-136629 is known. Alternatively, a rotary motor type electric brake device described in JP-A-10-504876 is known. Concerning the direct driven actuator type electric brake device, in addition to the electric brake device described in JP-A-60-136629 in which piezoelectric ceramic is used, it is possible to use an electric brake device in which a linear motor is used. On the other hand, concerning the rotary motor type electric brake device described in JP-10-504876, it is possible to use an electric brake device in which a rotary motion of an electric motor is converted into a linear motion by a feed screw mechanism and others.
On the other hand, an invention about electroactive polymer, the shape of which is changed by electric energy, is described in JP-A-2001-286162 and JP-A-2003-506858 and Non-patent Document 1 (Dielectric elastomer for MEMS and NEMS/EPAM technology and development from now on, by Masatake Chiba, pages 32 to 38, Vol. 18. No. 1 of “Electronic Packaging Technology” published by K. K. Gijyutsu Chosa Kai on Dec. 20, 2001). As described in the paragraph [0008] of the specification of JP-A-2001-286162, this electroactive polymer is composed and operated as follows. A pair of electrodes are provided on both sides of a plate-shaped elastomer polymer EP. When a voltage impressed between both the electrodes is adjusted, the elastomer polymer EP is expanded and contracted. JP-A-2001-286162 and JP-A-2003-506858 suggest the use of the above electroactive polymer for driving a portion to be driven of a valve device, a pump device, a robot or an artificial organ. Non-patent Document 1 introduces an artificial muscle in which an electroactive high polymer is used. According to the introduction of Non-patent Document 1, the performance of this artificial muscle is high in such a manner that the efficiency is high and the distortion is not less than 300%, the drive pressure is 8 MPa and the response time is not more than 0.5 ms.
In the electric brake device described in JP-A-60-136629 in which the actuator is made of piezoelectric ceramics, the following problems may be encountered. It is possible to increase a pushing force generated by the actuator. However, it is difficult to ensure a quantity of deformation of the actuator in the pushing direction. Therefore, when the pushing force is set at a high intensity at the time of applying the brake, it is impossible for the actuator to be deformed to be a sufficiently small size at the time when braking is not conducted, and the rotary member and the friction member are rubbed on each other. Therefore, it is difficult to obtain a high intensity of braking force while dragging of the rotary member and the friction member is being prevented. The occurrence of dragging is not preferable, because the running performance such as a fuel consumption or acceleration is deteriorated. On the other hand, in the case of an actuator in which a linear motor is used, it is possible to ensure a quantity of deformation, while the occurrence of dragging is being prevented. However, it is impossible to endure a sufficiently high pushing force.
On the other hand, in the rotating motor type electric brake device described in JP-A-10-504876, it is possible to make obtaining of a sufficiently high pushing force be compatible with obtaining of a sufficiently large quantity of deformation. However, the number of parts is so large that the weight and the volume are increased. Further, in the case of applying the brake, frictional engagement and elastic deformation are caused in a large number of portions. Therefore, the efficiency is so low that energy can not be effectively utilized.
In JP-A-2001-286162, JP-A-2003-506858 and Non-patent Document 1, various descriptions are made about the use of electroactive polymer characterized in that a quantity of deformation is large and the energy conversion efficiency is high. However, it is considered that this electroactive polymer is applied to an actuator used for a conveyer, an opening and closing device for a door and a movable portion of a robot (For example, refer to page 35 of Non-patent document 1).